This invention relates to a method of operating a measurement instrument.
In an optical time domain reflectometer (OTDR) a laser diode is periodically energized to launch light pulses into a fiber under test. Rayleigh backscattered light and Fresnel reflected light received from the fiber under test are applied to a photodetector. The photodetector provides a current that depends on the intensity of return light incident on the photodetector. This current signal is applied to a transimpedance amplifier, which converts the current signal to a voltage signal and amplifies it. The amplified voltage signal is sampled at predetermined times relative to the times of energization of the laser diode, and the sample values are measured using an analog-to-digital converter (ADC). The digital data values provided by the ADC are processed and are stored at memory locations that depend on the interval between launch of the interrogation pulse into the fiber under test and sampling of the output signal of the transimpedance amplifier. The stored sample values can be used to provide a display showing intensity of return light received from the optical fiber, expressed as attenuation of emitted light, as a function of distance from the laser diode.
The voltage signal is generally sampled using a sampling gate and a sample storage capacitor. The sampling gate is opened (rendered conductive) for a brief sampling interval to allow the storage capacitor to charge, and is then closed (rendered nonconductive). The voltage stored on the capacitor is measured by converting it to digital form under control of a conversion pulse that occurs in timed relationship to the operation of the sampling gate.
The return optical signal received from the fiber under test is composed of a low level backscatter signal and occasional brief pulses of very high magnitude due to reflections. Consequently, the return optical signal has a very large dynamic range. The dynamic range of the electrical signal that is sampled can be reduced relative to that of the return optical signal by designing the transimpedance amplifier so that it has a linear transfer function for small signal levels and a logarithmic transfer function for large signal levels. However, even then the dynamic range of the signal that is applied to the sampling system is substantial. Further, the complex nature of the overall transfer function of the transimpedance amplifier results in calibration being difficult and somewhat limited in accuracy.
The sampling efficiency of a sampling system is a measure of the relationship between the sample value provided at the output of the sampling system and the value of the input signal during the sampling interval. There are two aspects to the sampling efficiency of a sampling system that comprises a sampling gate and a storage capacitor, namely transfer efficiency and charging efficiency. Transfer efficiency of the sampling gate is a measure of the relationship between the voltage at the input of the sampling gate and the voltage at the output of the sampling gate during the sampling interval. Charging efficiency is a measure of the relationship between the voltage at the output of the sampling gate during the sampling interval and the voltage that exists on the storage capacitor when the sampling gate has closed at the end of the sampling interval. The transfer efficiency may be less than 100%, due to impedance in the arms of the sampling gate establishing a potential divider. A transfer efficiency less than 100% can be compensated by including a suitable gain stage downstream of the storage capacitor. The charging efficiency depends on the duration of the sampling interval. The duration of the sampling interval is limited by the bandwidth of the input signal, since the duration of the sampling interval is inversely related to the frequency of the input signal. The charging efficiency can also depend on the amplitude of the input signal, since the rate at which charge can be supplied to the capacitor through the sampling gate may be limited. Further, the charging efficiency can depend on the direction of the flow of current through the sampling gate. Moreover, if the amplitude of the input signal changes by a large amount between one sampling interval and the next, as is the case in an OTDR when consecutive samples represent a reflection event and backscattering, the measured value of the second sample can depend on the value of the first sample and therefore the sampling system is signal dependent.
The intensity of backscattered light received by an OTDR from a fiber under test is a function of the wavelength and intensity of the interrogation pulse. The intensity of the optical output provided by a laser diode is an approximately Gaussian function of wavelength and has a half-amplitude width of only a few manometers. The center wavelength is quite strongly dependent on temperature. Therefore, to obtain measurement repeatability in an OTDR it is necessary to maintain the laser diode at a constant temperature. This can be accomplished using a thermoelectric cooler that operates in a feedback loop with a temperature sensor. The operating temperature of the laser diode is established by a temperature level signal provided by the OTDR's internal controller. However, this does not guarantee measurement repeatability from instrument to instrument.